Synthesis gas contains carbon monoxide and hydrogen that can be further purified to produce hydrogen and carbon monoxide products or can be further reacted in such downstream chemical processes that, for example, involve the production of methanol or known gas to liquid processes for synthetic fuels by means of the Fischer-Thropsch process.
Synthesis gas is generated within a steam methane reformer by introducing a hydrocarbon containing feed, typically natural gas, into steam methane reformer tubes located in a radiant section of the steam methane reformer. The reformer tubes are packed with a catalyst that is used to promote the steam methane reforming reactions. Steam methane reforming reactions are endothermic and hence, heat is supplied to the reformer tubes to support the reactions by burners firing into the radiant section of the steam methane reformer. Synthesis gas can also be generated in a partial oxidation reactor by reaction between hydrocarbon and oxidant (e.g. oxygen) or in an autothermal reformer by reaction between hydrocarbon, oxidant and steam.
After a synthesis gas stream has been cooled, the steam and carbon monoxide content of the synthesis gas can be further reacted in a water-gas shift reactor to increase the hydrogen content of the synthesis gas.
An integrated steam generation system is located within the synthesis gas plant to produce the steam for the steam methane reforming reaction, for the water gas shift reaction and also, for export. The exported steam can itself constitute a valuable product that can effect the economic viability of the facility. Steam methane reformers typically have convective heat exchange sections that are connected to the radiant sections. The heated flue gases produced by the burners firing into the radiant section are passed through the convection section to raise steam and to superheat steam for the purposes outlined above. The steam generation system also utilizes heat exchangers both upstream and downstream of the water-gas shift reactor. In this regard, the synthesis gas stream generated in the steam methane reformer must be reduced in temperature to a level suitable for the water-gas shift reactor and consequently, a heat exchanger located upstream of the water-gas shift reactor both cools the synthesis gas stream and raises some of the steam. Since the water gas shift reaction is an exothermic process, the heat contained in the shifted stream is commonly utilized in heat exchangers located downstream of the water-gas shift reactor for the production of additional steam. All of such steam is routed to a steam header and then superheated in the convective section of the steam methane reformer.
The synthesis gas produced by the steam methane reforming reactions has a carbon dioxide content. After the water-gas shift reactor, the carbon dioxide content of the synthesis gas is further increased as a result of the reaction of the steam with the carbon monoxide. Separation of the carbon dioxide from the synthesis gas is often necessary for downstream processing of the synthesis gas, for example, in methanol production. Additionally, carbon dioxide itself is a valuable product. Thus, various carbon dioxide removal systems have been integrated with steam methane reforming facilities in order to at the very least to separate the carbon dioxide from the synthesis gas but also, to recover the carbon dioxide for sequestration purposes or for use as a value added product.
The carbon dioxide is separated from synthesis gas streams by absorbent systems. Such absorbent systems cam utilize solvents such as hot potassium carbonate, but more typically primary, secondary or tertiary amines for example monoethanolamine (MEA), diethanolamine (DEA), methldiethanolamine (MDEA), also mixtures of amines More recently MDEA and piperazine have been used as a solvent.
For example in U.S. Pat. No. 3,622,267, a synthesis gas under pressure of 40 atmospheres and containing 30 percent by volume of carbon dioxide is introduced into the bottom of an absorption column. An MDEA solvent is passed countercurrently to the synthesis gas in the absorber column to produce a column overhead that contains a low percentage of carbon dioxide. The rich liquid, that is produced as a liquid column bottoms of the absorber column, is then introduced to a packed column countercurrently with the carbon dioxide separated from the solvent in a regeneration column. The separated carbon dioxide is removed from the top of the packed column and a bottoms stream of solvent is introduced into an intermediate location of the absorber column. The remaining portion of the solvent is introduced into a regeneration column that is operated at a lower pressure than the absorbent column to produce the carbon dioxide and to regenerate the solvent which is reintroduced to the top of the absorption column. The regeneration column is reboiled with a boiler. In another embodiment, the bottoms liquid of the packed column is introduced into the regeneration column and the regenerated solvent is pumped back to the top of the absorption column.
U.S. Pat. No. 4,553,981 is an integrated process where carbon dioxide is scrubbed from a synthesis gas stream in a scrubbing column. The synthesis gas is produced by successive partial oxidation and water-gas shift reactor. The resultant scrubbed gas is then further treated in a pressure swing absorption unit to produce a hydrogen product. In this patent, the synthesis gas stream after having passed through the water-gas shift reactor has a temperature of about 430° C. This stream is passed to the scrubbing column that utilizes a physical absorbent to absorb the carbon dioxide. The physical absorbent after having been loaded with the carbon dioxide is flashed in flash tanks to regenerate the solvent so that it can be pumped back to the scrubber column.
U.S. Pat. No. 4,336,233 discloses another process that can be used to remove carbon dioxide as well as hydrogen sulphide, carbonyl sulphides and sulphur dioxide from a synthesis gas. In this patent, mixtures of piperazines and amines can be used as a solvent. Gas to be purified is introduced into an absorption column in which the carbon dioxide is absorbed within the solvent. The resultant rich liquid column bottoms of the absorption column is then expanded in a turboexpander and then introduced in a preliminary flash column to produce a flash gas and a purified column bottoms that is in turn introduced into a regeneration column in which the bottoms is reboiled to regenerate the solvent for use in the absorption column.
U.S. patent application 2003/0141223 discloses a similar process to that outlined above that uses an amine solvent in which loaded absorption liquid is first expanded to a pressure of from between about 1 to 2 bars absolute. The rich liquid is then partially regenerated in a first low-pressure expansion stage that utilizes a packed column and is then further expanded to a pressure from between about 1 to 2 bar (absolute) and introduced into a second low-pressure expansion stage. Second low-pressure expansion stage regenerates the absorbent for use in the absorption column.
In the integration of any of the processes discussed above with a steam methane reforming facility, a balance must be struck between the need to generate export steam with the desirability of recovering carbon dioxide because the temperature at which syngas is supplied to the absorption system, utilizing either a physical or a chemical adsorbent, will in large part determine the carbon dioxide recovery. Put another way, while it is desirable to supply the syngas to an absorption system at a higher temperature, a level can be reached at which steam production will necessarily suffer, thereby to have an effect on plant economics. However, where the synthesis gas is treated at a lower temperature, recoveries within the absorption column will tend to fall off to low levels of about 50 percent or less. This will occur where the synthesis gas stream is treated after the water-gas shift reactors and the downstream heat exchangers and the heat for the absorption process is solely supplied by the heat contained within the synthesis gas stream that typically has a temperature of anywhere between about 90° C. and about 150° C.
Hence, in order to boost recovery of the carbon dioxide, heat from another source that constitutes an addition to the heat of the synthesis gas stream itself must be used. The logical source of such heat addition is from steam. For example, in a paper, “Hydrogen Manufacturing Plant with CO2 Recovery and PSA Purification; Design and Operating Experience”, ERTC 10th Annual Meeting, Nov. 14-16, 2005, Vienna, Austria by Air Liquide and Haldor Topsoe, carbon dioxide is recovered in an MDEA wash downstream of the water-gas shift reactor heat exchangers and a water knock-out drum with the use of additional steam. Although details of the recovery process are not set forth in any detail in this paper it would appear that the carbon dioxide recovered is between 40 percent to 52 percent.
Another example of utilizing steam and a feed gas to provide the heat in an absorption process can be found in “Improve CO2 removal from ammonia plants”, by Nair, Hydrocarbon Processing August 2005. In this paper, a hot potassium carbonate-based carbon dioxide process is disclosed in which the regenerator column is reboiled by heat supplied by both low pressure steam and the feed gas through indirect heat exchange from bottom liquids.
The use of the synthesis gas at a lower temperature such as suggested by the Air Liquide, Haldor Topsoe paper presents a problematical integration in a retrofit situation because the absorption system must be located physically close to the existing equipment to avoid condensation of the water content of the synthesis gas which at the point of extraction of the synthesis gas is at or near its dew point. While, in a chemical facility, there exists many sources of inexpensive low pressure steam, the utilization of the same as a source of additional heat with the synthesis gas stream in separate heat exchangers will still require that the absorption facility be located close to the existing equipment to avoid condensation of the water content within the synthesis gas.
As will be discussed, the present invention, among other advantages, provides an integration in which carbon dioxide is recovered in an absorption system that is not constrained to be located particularly close to the point at which the synthesis gas is removed from the steam methane reforming system while allowing for a good economic balance of the requirement of consumption of steam, if any, from the steam generation system.